At atmospheric pressure or superatmospheric pressures, essentially in the range from 1 to 5 bar, propylene oxide and methanol can be separated by distillation only when a distillation column having a very large number of theoretical plates is used and a very high reflux ratio is set at the same time, owing to the entraining azeotrope.
These mixtures comprising propylene oxide and methanol result, e.g., from epoxidation processes where propene is reacted with a hydroperoxide such as hydrogen peroxide in the presence of methanol as solvent.
U.S. Pat. No. 5,849,938 discloses a process where propene is separated from methanol in a crude olefin epoxidation product by means of an extractive distillation wherein a relatively heavy polar solvent having hydroxy groups such as water or propylene glycol is used as the extracting solvent, propylene glycol being particularly preferred. According to this document of the prior art, the distillation column used ordinarily has from 20 to 60 theoretical plates, and the refluxidistillate ratio is generally in the range of from 5 to 15. According to the examples, a typical ratio is 9. Typical bottoms temperatures are in the range of from 90 to 120° C., the pressure under which distillation is carried out being from 0.55 to 3.44 bar. According to the example, a preferred bottom pressure of the distillation column is 2.76 bar and therefore well above standard pressure. As typical propylene oxide fractions, fractions are obtained comprising 300 or 1,500 ppm of methanol. The bottoms fractions obtained according to the examples comprise up to 6,300 ppm of propylene oxide.
U.S. Pat. No. 6,500,311 B1 discloses a process wherein a separation of methanol and propylene oxide takes place. As extracting solvent, a non-polar solvent, namely a C7-C9 hydro-carbon such as n-octane is used.
It is an object of the present invention to provide a method of separating propylene oxide from methanol which, compared to the processes described in the prior art, has an improved energy balance and, additionally, leads to top streams and bottoms streams having a lesser degree of impurity with regard to methanol and propylene oxide, respectively.
It is a further object of the present invention to provide a method of separating propylene oxide from methanol in which a cheap extracting solvent is employed which simultaneously allows for milder distillation conditions than those described in the prior art.
It is still another object of the present invention to provide a method of producing propylene oxide in the course of which propylene oxide is separated from methanol wherein this separation has the above-mentioned advantages thus rendering the method for producing propylene oxide energetically and also with respect to the purity of the distillation fractions advantageous over the prior art.